Furnace control circuit

ABSTRACT

A furnace control circuit includes a solid state digital logic circuit which controls a solid state ignition, burner control and lockout circuits to control the firing of a gun type oil furnace or its equivalent. The logic circuit receives signals from a burner motor sensing circuit, the furnace control thermostat and a flame detection circuit. The logic circuit responds to signals from these circuits to detect failure conditions and respond to prevent actuation or deactuate the furnace burner motor in the event that ignition has not been achieved when called for or other failures occur. The logic circuit includes inhibit circuit means for selectively inhibiting a low frequency digital oscillator providing a pulse after a predetermined delay period to control the burner motor lockout circuit in the event a failure mode of operation is sensed.

United States Patent Juskewicz, Jr.

FURNACE CONTROL CIRCUIT Inventor: Joseph A. Juskewicz, Jr., Grand US. Cl. 307/117, 431/69 Int. Cl. F23n 5/00 Field 01 Search 307/116, 117; 431/24, 25,

[56] References Cited UNITED STATES PATENTS 3,624,407 11/1971 Bauer 431/42 3,732,433 5/1973 Lourigan 307/117 Primary E.ramiIierRobert K. Schaefer Assistant Examiner-M. Ginsburg Attorney, Agent, or FirmPrice, Heneveld, Huizenga & Cooper [5 7] ABSTRACT A furnace control circuit includes a solid state digital logic circuit which controls a solid state ignition, burner control and lockout circuits to control the firing of a gun type oil furnace or its equivalent. The logic circuit receives signals from a burner motor sensing circuit, the furnace control thermostat and a flame detection circuit. The logic circuit responds to signals from these circuits to detect failure conditions and respond to prevent actuation or deactuate the furnace burner motor in the event that ignition has not been achieved when called for or other failures occur. The logic circuit includes inhibit circuit means for selectively inhibiting a low frequency digital oscillator providing a pulse after a predetermined delay period to failure mode of operation is sensed 10 Claims, 2 Drawing Figures +v Ioo THERMOSTAT I04 22 7 96 I02 DIGITAL l l BURNER MOTOR BI-DIREcTIoNAL coNrRoL 94 LATCH ClRCUlT 09 2| 50 I /ll 82 l 84- '3 [32 II FLAME SENSING a 97: "6 MOTOR VOLTAGE .41 BURNER 2o IGNITIoN CONTROL 114 SENSING MOTOR CIRCUITS "2N CIRCUIT l Io I34 ii"; 1 DIGITAL 24 W 4 0 5 1 OSCILLATOR Ho 86 I ENABLE/INHIBIT i P 62 ll? 121 LOW-VOLTAGE 159V DIGITAL POWER SUPPLY OSCILLATOR l 36 3| [24 122 lb II I G -1 I DIGITAL LOCK-OUT u njR 125 LATCH CRCUIT 5/ l2 32 20 vAc IGNITION I I2 CIRCUIT f 43 v 42 |3 FURNACE 55 4o SYSTEM 54 SWITCH IGNITIoN SPARK mm a m m MAR 1 8 1925 PATEN 2 on m m m oi NE vn w n H 1E5 @Q n 4 n 1 AIL. mm: H l l I k: 02 .v mung; 1m mO w A mm mg mm mo. mm om 3 Q. h M 9 .1: i l R Nb m FURNACE CONTROL CIRCUIT BACKGROUND OF THE INVENTION The present invention relates to an improved furnace control circuit.

Furnaces which employ a burner motor for pumping oil to a burner within the furnace and a spark ignition circuit for igniting the oil require safety devices for assuring that in the event ignition is not achieved as commanded by the thermostat, the burner motor will be shut off to prevent further pumping of oil into the furnace and the resultant potential fire hazard. Existing electrical control circuits for oil furnaces employ thermally responsive type relays or switches. Unfortunately, the thermally responsive electromechanical devices are slow reacting and are themselves subject to failure. Typically, they require additional back up safety devices and circuits resulting in a relatively complex control system both costly and prone to failure.

In an attempt to reduce the complexity, cost and failure rate of such circuits, attempts have been made to manufacture a hybrid control system employing, at least partially, more reliable solid state electrical devices to provide switching functions. U.S. Pat. No. 3,624,407 issued to Frederick T. Bauer on Nov. 30, 1971 represents one such system. The hybrid systems, however, still use specially designed electromechanical and/or thermally responsive circuit elements in addition to the solid state switching devices employed. Thus, although the recent efforts are partially successful in eliminating several of the electromechanical devices previously subject to relatively high failure rates as compared to the solid state devices, currently available furnace control circuits still employ failure prone specially designed devices which increase the cost of the circuits and reduce their reliability.

There exists a need, therefore, for'an improved furnace control system employing full solid state circuitry and eliminating thermally responsive electromechanical devices.

SUMMARY OF THE INVENTION The present invention fulfills such a'need by providing a control circuit employing digital solid state circuitry which eliminates thermally responsive electromechanical components to provide fail-safe control of the furnace burner motor or ignition circuit. Systems embodying the present invention employ a logic circuit which receives signals indicating the existence of a burner flame and the actuation of the burner motor together with the command signal from the thermostat. The logic circuit responds to these signals and includes a low frequency oscillator to provide a control signal applied to a lockout circuit which deactivates the burner motor in the event a failure mode is detected. The control system provides normal mode operation for furnace ignition and shut off as commanded by the furnace thermostat associated with the furnace.

It is an object of the present invention, therefore, to provide an improved furnace control system providing fail-safe operation.

Another object of the present invention is to provide a furnace control system with a digital logic circuit for detecting failure modes of operation and providing a control signal to deactivate the furnace in the event a failure occurs.

Still a further object of the present invention is to provide a digital logic circuit for a furnace control system which includes a controlled oscillator providing a time delayed signal for controlling the fuel fed to the burner of a furnace in the event fuel ignition does not occur during a predetermined delay period.

An additional object of the present invention is to provide a furnace control circuit with means for detecting the operational condition of a burner motor in an oil burning furnace.

Yet another object of the present invention is to provide an improved lockout circuit which permits normal furnace operation and deactivates the furnace in detected failure modes of operation to provide fail-safe operation.

These and other objects of the present invention will become apparent upon reading the following description thereof together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block electrical circuit diagram partially in schematic form showing the circuitry of the present invention; and

FIG. 2 is a schematic electrical circuit diagram partially in block form showing the individual circuit elements employed in the circuit of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The furnace control circuit 10 embodying the present invention is shown in FIG. 1 and includes a pair of input terminals 12, 12 for coupling a source of volt AC 60l-lz power to conductors 15 and 13 respectively. A conventional power on-off switch 14 is coupled in series with conductor 13 coupled to the terminal 12' for actuating and deactuating the system. The control circuit 10 may also include a suitable fuse or circuit breaker coupled to the input power leads 13 or 15. e

The system shown in FIG. 1 is employed in a gun typ oil furnace frequently used in mobile homes although it has equal applicability to other types of oil furnaces or even gas furnaces. In the preferred embodiment, the furnace includes a burner motor 20 having power receiving terminals 21 and 24 and which actuates a pump for pumping fuel from a fuel line into the burner of the furnace (not shown). The current path for supplying operating current to the burner motor includes a solid state AC switch 22 such as a triac selectively coupling conductor 13 of the power supply line to terminal 21 of the motor. The current path for motor 20 is completed by means of a lockout circuit 30 having one terminal 31 coupled to the motor terminal 24 and a second terminal 32 coupled to conductor 15.

Lockout circuit 30 is selectively actuated by the receipt of a signal applied to a control input terminal 36 which, in the preferred embodiment, is a negative going pulse shown in the waveform diagram 33 (FIG. 2) adjacent terminal 36. Signal 33 actuates the lockout circuit deactivating the burner motor and an ignition circuit 40 in the event a failure mode occurs.

Circuit 30 includes a lockout relay 34 having a control coil 35 coupled to a low voltage +V supply by means of a resistor 37. The opposite terminal of coil 35 is returned to ground by means of a second resistor 38. Associated with relay 34 is a pair of switch contacts 39 which are held in a normally closed position to com- 3 plete the burner motor and ignition circuit current paths when current flows through relay coil 35. Thus, in the event of a loss of power, the switch contacts 39 will open to deactivate the burner motor for this failure condition.

In addition, however, the lockout circuit is actuated in response to a plurality of other failure conditions. This is achieved by a current shunting transistor having a base terminal 25b coupled to'the control input terminal 36, an emitter terminal 252 coupled to one of the coil terminals of coil 35,and a collector terminal 250 coupled to the remaining terminal of coil 35. Transistor 25 is an NPN transistor with sufficient current capability to shunt sufficient current around coil to effect opening of contacts 39 in the event a negative going control signal 33 is applied to the base of the transistor which occurs in the event of a failure mode as discussed below. By providing the switch contacts 39 in series with the burner motor and controlling the switching of contacts 39 in this manner, both power supply failure and detected failure modes provide failsafe control for the furnace. I

The furnace further includes an ignition circuit 40 coupled to terminal 31 of the lockout circuit by means ofan input terminal 41 and to the opposite side of the power line by means of terminal 42. The ignition circuit has .a control input terminal 43 for receiving control signals and responding thereto for actuating an ignition transformer 50 with primary and secondary windings 52 and 54 respectively. A spark gap 55 coupled across the secondary winding provides an ignition spark in the vicinity of the burner jets for igniting fuel. I

As seen in FIG. 2, the ignition circuit is also deactuated by the lockout circuit since the input terminal 41 is the ground return for the ignition circuit which is selectively coupled to the power supplying conductor 15 through the lockout switch contacts 39. Thus, when the lockout circuit is'actuated to shut thefurnace off during a failure mode, the burner motor 20 and the ignition circuit 40 are both deactuated.

As seen in FIG. 2 the ignition circuit 40 is an SCR pulsed oscillator in which a power supplying capacitor 44 charged in the polarity shown in FIG. 2 is periodically discharged by'the firing of SCR 45 to provide a pulsing current to the primary winding 52 of the ignition transformer 50.. Capacitor 44 is charged during positive half cycles of each 60Hz interval (measured from the conductor 13 relative to conductor 15) by a half wave current supplying circuit including resistor 46, rectifying diode 47 and choke 48 which are serially coupled between input terminal 42 and capacitor 44.

The series combination of resistor 49 and capacitor 51 coupled between the anode and cathode of SCR 45 provide, in combination with capacitor 53 shunting resistor 49, conventional over voltage protection for the SCR. SCR 45 is self-extinguishing during alternate half cycles of ignition transformer current when capacitor 44 is discharged. Radio frequency'interference suppression is provided by means of choke 56 and series coupled capacitor 57 coupling thefloating common conductor 58 coupled to the cathode of SCR to earth ground 59. SCR 45 is periodically triggered by a unique gate circuit comprising a series RC circuit coupled across the anode and cathode of-the SCR and consisting of resistor 16 and capacitor 17. The junction of these components is coupled to the anode of a silicon unilateral switch (SUS) 18 having its cathode coupled to the .gate terminal 45g of SCR 45through a current limiting resistor 19 and a pairof switch contacts 231 associated with a reed relay 29. Relay 29 includes a relay coil 26 in turn actuated by ignition control circuit 80-as described below.

When switch contacts 231' are closed, which occurs when ignition is called for, capacitor 17 will discharge through the SUS 18 when a predetermined voltage level has been reached to apply a positive going signal to the gate of SCR 45 thereby tiring the SCR. Conventional gate protection for SCR 45 is provided by means of resistor 27 and diode 28 coupled-as shown in H6. 2. It is seen that when the lockout circuit is actuated and contacts 39 are open, the ignition circuit 40 is effectively disconnected from the current power supplying conductors l3 and 15 thereby deactuating the circuit.

The control circuit 10 includes a low voltage power supply 60 which is coupled to the power line, as seen in FIG. 1,.and provides a low direct voltage output +V at output terminal 62-thereof. Voltage +V is employed to .actuate the various digital and other circuit elements of the control circuit as well as the thermostat 70.

Power supply 60 comprises a conventional full wave power supply utilizing a transformer 61 having a grounded center tapped secondary 63 and a pair of rectifying diodes 64 and 65 with anode terminals coupled to opposite ends of the secondary. Diodes 64 and 65 have their cathode terminals coupled to a filter'circuit comprising resistor 66 and capacitor 67. Voltage regulation is provided by an. avalanche diode 68 coupled across filter capacitor 67 as shown in the figure.

Thermostat 70 includes an input terminal 72 coupled to the voltage supply +V and an output terminal 73 coupled to ground by means of an anticipation current shunt resistor 74 which is required since the current through the thermostat 70 without resistor 74 would be very low due to the solid state circuitry employed and the conventional heat anticipator included in the thermostat would not-operate. As seen in FIG. 2, thermostat 70 comprises a thermally responsive switch 7 l with one contact associated with terminal 72 coupled to +V and its remaining contact associated with terminal 73 coupled to a control input terminal 82 of the flame sensing and ignition control circuit 80. i

Circuit includes, as seen in FIG. 2, a voltage divider comprising a fixed resistor 81 and a light or heat responsive resistor 83 coupled in series with resistor 81, the series combination being coupled from terminal 82 to ground. Resistor 83 comprises, in the preferredembodiment, a cadmium sulfidecell positioned near the burner (i.e., guns) of the furnace to be responsive to light from ignition of the fuel to provide a greatly reduced resistance when the fuel is ignited.

The junction of resistors 81 and 83 is coupled to the base terminal b of transistor 85 which is an NPN transistor having a collector terminal 850 coupled to 1 output terminal 84 and to the -+V supply voltage through the relay coil 26. The emitter terminal 85e'of transistor 85 is coupled to ground. Transistor 85 will be positive voltage to the base of transistor and when no light is striking resistor 83 (i.e., prior to ignition). Once ignition has been achieved, the resistance of 83 drops sufficiently low to render transistor 85 nonconductive causing the collector voltage at terminal 84 to raise to a 1 logic state. This ignition representative signal (86) is applied to a bidirectional latch circuit 90 at input terminal 94 thereof. Circuit 90 also includes an input terminal 92 coupled to the output terminal 73 of the thermostat via resistor 75 to receive a heat command signal therefrom.

Latch 90 comprises a first NAND gate 91 cross coupled to a second NAND gate 93 as'seen in FIG. 2. Gate 91 includes one input terminal coupled to terminal 92 and an output terminal coupled to an output terminal 96 of the latch and further coupled to one of the input terminals of NAND gate 93. The remaining input terminal of gate 93 is coupled to input terminal 94 while its output terminal is coupled to the remaining input terminal of gate 91.

Latch 90 operates in a conventional manner to apply a logic 0 signal at outupt terminal96, as indicated by waveform 97 in FIG. 1, which is employed as a burner motor control signal applied to terminal 102 of the burner motor control circuit 100. A logic 0 actuates the burner motor when the thermostat switch closes and no light is detected by sensor 83. When the thermostat switch opens, the latch switches to a logic 1 output state inhibiting burner control circuit as now described.

Burner motor control circuit 100 comprises a PNP transistor 103 having its base terminal 103b coupled to input terminal 102 and a collector terminal 1030 coupled to ground via resistor 105. Emitter terminal l03e is coupled to the +V supply through the coil 106 of reed relay 107. Relay 107 includes a pair of switch contacts 108 actuated to close by collector current flowing through coil 106 to short the output terminals 104 and 109 of the control circuit 100. These output terminals in turn are coupled between gate terminal 23 and the cathode 23' of triac 22 triggering the triac into conduction allowing current to flow through the burner control motor 20.

A current limiting resistor 197 is coupled between output terminal 104 and gate terminal 23 of the triac while a conventional transient over voltage RC circuit comprising a series capacitor 98 and resistor 99 is coupled across the triac. Triac 22 will be rendered conductive' whenever the switch contacts 108 of relay 107 are closed to actuate the burner motor and will be rendered nonconductive whenever switch contacts 108 open in response to a"1 control signal from the latch circuit 90.

The furnace control circuit 10 further includes a logic circuit 110 receiving input signals from circuits 80 and 90 and from a motor voltage sensing circuit 130 to detect various failure modes of operation and generate a control signal for selectively actuating lockout circuit 30 in the event a failure condition exists. Circuit 110 includes, as seen in FIG. 1, a digital oscillator enable/inhibit circuit having an output terminal 117 coupled to a control terminal 121 of a digital oscillator 120. Output terminal 122 of oscillator is coupled to an input terminal 124 of a digital latch circuit 125. Latch 125 includes an output terminal 126 which is coupled to the control input terminal 36 of the lockout circuit 30 as discussed above.

The logic circuit includes an input terminal 112 coupled to output 84 of the circuit 80, input terminal 114 coupled to output terminal 96 of circuit 90, and an input terminal 116 coupled to output terminal 136 of circuit 130. Before discussing the logic circuit in detail, the motor voltage sensing circuit is described.

Circuit 130 includes, as best seen in FIG. 2, an input terminal 132 coupled to terminal 21 of the burner motor 20 and a second input terminal 134 coupled to terminal 24 of the motor. When the motor is actuated, the alternating current voltage across the motor is applied to terminals 132 and 134 and is rectified by means of a diode 131 and filter circuit comprising capacitor 135 and resistor 137. The direct voltage across capacitor 135 is limited by an avalanche diode 138 coupled in parallel to the capacitor. Diode 131 is employed to provide a DC current through the coil 139 of a reed switch relay 140 having reed switch contacts 142 associated with coil 139 and coupled from output terminal 136 to ground. When motor 20 is actuated, therefore, the voltage across the motor is converted by circuit 130 to a logic 0" signal at output terminal 136 used as a control signal applied to input terminal 116 of the logic circuit 110 for providing information indicating the operational status of motor 20.

The enable/inhibit circuit 115 of logic circuit 110 includes a first NAND gate 145 (FIG. 2) having a pair of input terminals interconnected to form an inverter circuit. The input terminals are coupled to +V by means of a resistor 146 and to the switch contacts 142 of relay 140 by means of interconnected terminals 116 and 136. The output terminal 147 of gate 145 will normally be at a logic low level and will switch to a logic high level only when switch contacts 142 are closed when motor 20 is running.

The output terminal 147 of gate 145 is coupled to one input terminal 148 of a second NAND gate-150 having a second input terminal 149 coupled to the out put of latch circuit 90 by means of interconnected terminals 114 and 96. An output terminal 152 of gate 150 is coupled to a first input terminal 154 of a third NAND gate 158 in circuit 115. The remaining input terminal 156 of gate 158 is coupled to input terminal 112 of circuit 110 while its output terminal is coupled to output terminal 117 of circuit 115.

As will be discussed below under OPERATION, a logic 0 is developed by circuit 115 and applied to the digital oscillator 120 to inhibit the oscillator during normal operation and a logic 1" is developed by the inhibit circuit 115 to actuate the oscillator in the event a failure mode is detected. These signal conditions are represented by waveform 159 in FIG. 1 shown adjacent output terminal 117 of circuit 115. It is noted here that the various NAND gates of circuit 115 and the remaining circuits are suitably coupled to the power supply +V in a conventional manner to receive operating power therefrom.

Continuing now with the description of the logic cir cuit 110 and specifically, the oscillator circuit 120 used for developing a delayed pulse for actuating the lockout in the event a failure mode exists and has not been corrected after a predetermined time, reference is again had to FIG. 2 showing the specific circuit details. The oscillator 120 comprises a low frequency astable multivibrator comprising first and second NAND gates 160 and respectively. Gate 160 has one input terminal coupled to the control input terminal 121 of the oscillator to receive control signal 159 from the ena ble/inhibit circuit ,1 15. Gate 160 includes an output terminal 162 coupled to an input terminal 164 of the second NAND gate 165. The remaining input terminals of gate 165 are also coupled to control input terminal 121.

The output terminal 166 of gate 165 is coupled to. input terminals 163 of gate 160 by means of a capacitor 167 coupled between terminals 166 and 163. A resistor 168 is coupled from the junction of capacitor 167 and terminals 163 to output terminal 162 of gate 160. A duty cycle controlling diode 169 is coupled from input terminal 164 of gate 165 to the junction of capacitor 167 and resistor 168.

This oscillator configuration provides a very low frequency free running multivibrator having a narrow output pulse as compared to the periodicityof the oscillator. In the preferred embodiment, resistor 168 was approximately 300 megaohms while capacitor 167 had a value of 0.1 MFD. Thus, the period of the oscillator was approximately 30 seconds while the provision of diode 169 assures the negative going output pulses have a duration of approximately 5 milliseconds.

The output signal waveform 170 present at the terminal 122 of oscillator 120 is shown in FIG. 1. When actuated, oscillator 120 generates a negative going output pulse 170 approximately every 30 seconds although the first occurring pulse will actuate latch circuit 125 to a continuously latched position providing a controlsignal to the lockout circuit 30 for deactuating the burner.

During normal operation, oscillator 120 is inactivated permitting the burner motor to run through the normally closed switch contacts of the lockout circuit.

Latch circuit 125 comprises, as best seen in FIG. 2, a first NAND gate 172 having an input terminal 173 coupled to input terminal 124 of the'circuit. Terminals 122 and 124 are interconnected. A transient spike bypass capacitor 174 is coupled from terminal 173 to ground. The output terminal 175 of gate 172 is coupled to the input terminals of three parallely coupled gates 176, 177 and 178, each having interconnected output terminals forming a single output terminall79. Terminal.179 is coupled to input terminal 171 of gate 172 to form a conventional latch but providing ample drive to control shunt transistor 25 of the lookout circuit. A pull down resistor 180 is coupled to the input terminals of gates 176, 177 and 178 to ground.

Latch 125 will normally provide a logic 1 output signal but will latch into a logic output signal in the event a negative going pulse is applied to input terminal 173 of gate 172 which occurs when oscillator 120 has been enabled for at least one period of oscillation. This will occur only when a failure mode has been detected and the oscillator continuously enabled for the predetermined period-Once latch 125 has been actuated by a pulse from oscillator 120, it will remain in its latched condition providing a logic 0" output signal (waveform 33) in turn actuating transistor 25 to shunt relay coil current to open lockout contacts 39 until the power supply 60 has been deactuated by opening the system switch 14. Resistor 180 assures that the output of latch 125 will be at a logic 1 when reset.

Having described the operation of the individual circuits constituting the control circuit for-the furnace and the signals at the various input and output terminals thereof, a description of the operation of the system to 8 provide operation of the furnace during normal and fail-safe failure modes of operation isnow presented.

OPERATION input terminals since the thermostat switch 71 is open.

When the temperature drops below the preset temperature of the thermostat, however, or if the thermostat temperature is increased over the ambient temperature sensed by the thermostat, a positive voltage will be applied through the now closed switch contacts 71 of the thermostat to the flame sensing circuit 80. Also,

a positive voltage corresponding to a logic 1 signal will be applied to input terminal 92 of latch 90. Since no flame has yet been detected, transistor will be rendered conductive'drawing current through the coil 26 of the reed relay 29 causing switch contacts 231 to close thereby actuating the ignitor circuit 40 as discussed above. The resulting logic 0 signal at the collector of transistor 85 is applied to terminal 94 of circuit 90. Gate 93 responds to this signal to apply a logic 1. to one input of gate 91. Simultaneously, the logic 1 applied to terminal 92 of latch will cause the generation of a logic'0 output signal at terminal 96 thereof causing the burner motor circuit to activate triac 22 thereby actuating the burner motor 20.

As soon as the flame has been established, it will be detected by resistor 83causing its resistance to drop to a relatively low value lowering the base voltage applied to transistor 85 until it is rendered nonconductive. The current path through coil 26 is, therefore, interrupted opening associated contact 231 thereby deactivating the ignition circuit 40. At this time, a logic 1 signal is applied at terminal 94 of latch 90 which does not affect the output signal at terminal 96 since the input at 92 remains in a logic 1" state. Thus, the burner motor will continue operating to supply fuel to the now fired furnace until the ambient temperature has increased to open the thermostat contacts.

It is noted that during normal operation when a heat command signal is first transmitted by thermostat 70, the oscillator will be started due to a logic 1 present at terminal 117 of gate 158 applied to terminal 121 of the oscillator. This results since transistor 85 conducts and applies a logic 0" to terminal 156 of gate 158. As soon as the flame is detected, transistor 85 is rendered nonconductive thereby applying a logic 1 to terminal 156 of gate 158 to apply a logic 0 at terminal 117 to inhibit the oscillator which is desired if ignition occurs prior to the predetermined time delay which is the period of the oscillator.

One of the many failure conditions that may exist is that after a heat command has been transmitted and the motor and igniting circuits actuated, ignition is not achieved and no flame is detected. In such instance, transistor 85 will remain conductive thereby continuing the operation of oscillator until after the predetermined delay period. The oscillator generates a pulse 170 which, when applied to latch 125, applies a to transistor 25 actuating the lockout circuit as discussed above. This is the most common failure condition and fail safe operation is assured under this condition which can occur, for example, if the oil supply is depleted, if the spark gap becomes inoperative, or if the ignition circuit fails.

Another failure condition which may occur is that once the thermostat has opened and the present temperature reached, the burner motor continues to run. This may result if triac 22 fails typically by shorting, transistor 103 shorts, reed relay contacts 108 remain closed, or due to other circuit failures. In this situation, the motor voltage sensing circuit will continuously apply a logic 0 to the input terminal 116 of gate 145 which continues applying a logic 1 signal to terminal 148 of gate 150. At this time, however, latch circuit 90 will apply a logic 1 to input'terminal 114 of gate 150 instead of a logic 0 as occurred with the thermostat switch closed. The resulting logic output 0 applied to terminal 154 of gate 158 will cause 'a logic 1 to be applied to the oscillator for enabling oscillator 120. After the predetermined delay period, therefore, actuation of lockout circuit 30 will occur to deactuate the burner motor.

Another failure mode which may occur is the loss of low voltage power from supply 60. It is noted here that if all line power is lost, the burner motor will be inactivated. If the low voltage power only, however, is lost, lockout automatically occurs since the lockout relay 34 is actuated through the low voltage supply to hold the contacts closed and .will open in the event of loss of power thereto.

Finally, in the event that as the thermostat switches are closed thereby providing a heat command signal, should the flame detection cell 83 detect light due to the existence of a flame, fire or ambient abnormal light leakage into the furnace, the motor will not be actuated since transistor 85 will not be rendered conductive to apply a logic 0 to 'the motor control circuit thereby preventing the switching of latch 90 to apply an enable signal to circuit 100 and maintaining triac 22 in a nonconductive state.

It is noted that the control circuit will also include a normal stack temperature switch and motor overload switches in series with the burner motor which are conventional and not shown. Such safety devices will include overheat sensors which, in the event the thermostat contacts lock shut, turn off the furnace once the stack temperature has reached a predetermined value. The primary control circuit disclosed herein, however, accommodates for failure modes of operation which can occur due to failure of components within the control circuit itself or other abnormal conditions.

The lockout circuit does not rely on thermally responsive devices and, therefore, in the event of a temporary nonrecurring failure, can be immediately reset without waiting for a cooling off period as required in many prior art devices. Failure of the shunting transistor 25, if it ever occurs, will almost certainly be the shorting of the transistor which will cause the lockout circuit to be actuated.

'In the preferred embodiment, the following circuit element values were employed:

Reference Numeral Value capacitors: 17 .01 M FD 44 .47 MFD SI .39 MFD 53 .Ol MFD 57 .047 MFD 98 .39 MFD 135 33 MFD 174 .01 MFD resistors: 16 150 K Ohms 19 l K Ohms 27 l K Ohms 37 56 Ohms 38 3.9 Ohms 46 10 Ohms 49 l K Ohms 74 40 Ohms 75 2.7 K Ohms 8] 10 K Ohm 83 CdS cell 97 100 Ohms 99 750 Ohms I00 Ohms 146 2.7 K Ohms 2.7 K Ohms diode: 18 GE. 2N4987 or 2N4990 It will become apparent to those skilled in the art that various modifications to the circuitry disclosed and described in the preferred embodiment may be made without departing from the spirit or scope of the invention as defined by the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. For use in a furnace installation including a burner and a source of electrical power, a furnace control system comprising:

fuel supplying means selectively coupled to said source of electrical power for supplying fuel to said burner;

selectively actuated ignition means associated with said burner for igniting fuel therefrom;

first means coupled to said fuel supplying means for detecting when said supplying means is actuated to supply fuel to said burner;

second means positioned in proximity to said burner for detecting a flame;

means for generating a temperature command signal for actuating said fuel supplying means and said ignition means for actuating the furnace;

a control circuit coupled to said first and second detecting means and to said generating means for providing a control signal in response to predetermined detected conditions indicating the existence of a failure condition including loss of power to said fuel supplying means; and

a lockout circuit coupled from said fuel supplying means and said ignition means to said source of electrical power and further coupled to said control circuit and responsive to control signals from said control circuit to deactuate said fuel supplying means and said ignition means in the event a failure is detected.

2. The system as defined in claim 1 wherein said control circuit includes a selectively actuated oscillator which is actuated upon the detection of a failure condition to provide said control signal after a predetermined delay period.

3. The system as defined in claim 1 wherein said fuel supplying means includes electrically actuated means for controlling the flow of fuel to said burner and wherein said lockout circuit includes a controllable switch serially coupled to said electrically actuated means to open the electrical current path therethrough when a failure condition occurs,

4. The system as defined in claim 3 wherein said lockout circuit comprises:

a relay including a relay coil with said switch having contacts associated therewith;

means for supplying power to said coil for closing said switch contacts during normal furnace operation; and

a transistor having a collector-to-emitter current path coupled in parallel to said coil and including a base terminal coupled to said control circuit to receive control signals therefrom and responsive to said control signals to conduct to shunt current from said relay coil to open said switch contacts.

5. For use in a furnace control system employing an electrically operated fuel supplying means, the combination comprising;

1 first means for sensing a furnace flame and second means for sensing the actuation voltage of said electrically operated fuel supplying means;

, solid state circuit means coupled to said first and second sensing means for detecting predetermined failure conditions and including a low frequency oscillator for providing a control signal representative thereof after a predetermined time delay; and

lockout circuit means including controlled switch means coupled in series with the fuel supplying means, said lockout circuit means further including a control element coupled to said solid state circuit means and responsive to said control signal therefrom to actuate said controlled switch means for opening the operating current path of said fuelsupplying means.

6. The combination as defined in claim 5 wherein said controlled switch means comprise a relay with switch contacts held closed when current is applied to a relay coil associated with said relay during normal furnace operation, said switch contacts coupled in series with said fuel supplying means.

7. The combination as defined in claim 6 wherein said control element comprises a transistor having base, collector and emitter terminals with said base terminal coupled to said solid state circuit means and said collector and emitter terminals coupled to opposite ends to said relay coil such that when said transistor is rendered conductive by said control signal applied to said base terminal, collector current flowing in said transistor shunts current from said relay coil opening said switch contacts in series with said fuel supplying means.

8. For use in an oil furnace with a furnace burner, a

burner motor and ignition means, a control circuit comprising:

voltage detecting means coupled to said burner motor for detecting the existence of operating 5 power applied thereto;

flame detecting means associated with the-furnace burner for providing a signal representing the existence or nonexistence of ignition; means for generating a heat command signal;

i a digital logic circuit coupled to said flame detecting means, to said voltage detecting means and to said generating means to provide a failure control signal after a predetermined delay period in the eventa failure condition occurs; and

lockout circuit means coupled to said burner motor and to said logic circuit for receiving signals from said logic circuit and responsive to failure control signals therefrom for interrupting the current path for said burner motor in the event a failure occurs.

9. The control circuit as defined in claim 8 in combination with a thermostat switch for actuating a burner motor control circuit and wherein digital logic circuit includes:

gate circuit means having a first input terminal coupled to said voltage detecting means, a second input terminal coupled to said flame detecting means, a third input terminal coupled to said burner motor control circuit and an output terminal for developing a pulse at said output terminal in the event a failure condition occurs; and pulse generating means including a control input terminal coupled to said output terminal of said gate circuit means for providing an output pulse for actuating said lockout circuit after a predetermined time delay upon receipt of a pulse from said gate circuit means. 10. A protection circuit for preventing the operation of an ignition circuit associated with a furnace burner in the event a furnace control fails to supply the burner motor with operating power when heat is called for by the systems thermostat, said circuit comprising:

voltage sensing means coupled to said burner motor to sense the voltage of the operating power supplied thereto; i logic circuit means coupled to said voltage sensing means and to said furnace control for developing an output signal in the absence of power applied to said burner motor when said thermostat actuates to actuate said burner motor; and a lockout circuit coupled to said ignition circuit and to said logic circuit means and responsive to saidoutput signal to deactuate said ignition circuit. 

1. For use in a furnace installation including a burner and a source of electrical power, a furnace control system comprising: fuel supplying means selectively coupled to said source of electrical power for supplying fuel to said burner; selectively actuated ignition means associated with said burner for igniting fuel therefrom; first means coupled to said fuel supplying means for detecting when said supplying means is actuated to supply fuel to said burner; second means positioned in proximity to said burner for detecting a flame; means for generating a temperature command signal for actuating said fuel supplying means and said ignition means for actuating the furnace; a control circuit coupled to said first and second detecting means and to said generating means for providing a control signal in response to predetermined detected conditions indicating the existence of a failure condition including loss of power to said fuel supplying means; and a lockout circuit coupled from said fuel supplying means and said ignition means to said source of electrical power and further coupled to said control circuit and responsive to control signals from said control circuit to deactuate said fuel supplying means and said ignition means in the event a failure is detected.
 2. The system as defined in claim 1 wherein said control circuit includes a selectively actuated oscillator which is actuated upon the detection of a failure condition to provide said control signal after a predetermined delay period.
 3. The system as defined in claim 1 wherein said fuel supplying means includes electrically actuated means for controlling the flow of fuel to said burner and wherein said lockout circuit includes a controllable switch serially coupled to said electrically actuated means to open the electrical current path therethrough when a failure condition occurs.
 4. The system as defined in claim 3 wherein said lockout circuit comprises: a relay including a relay coil with said switch having contacts associated therewith; means for supplying power to said coil for closing said switch contacts during normal furnace operation; and a transistor having a collector-to-emitter current path coupled in parallel to said coil and including a base terminal coupled to said control circuit to receive control signals therefrom and responsive to said control signals to conduct to shunt current from said relay coil to open said switch contacts.
 5. For use in a furnace control system employing an electrically operated fuel supplying means, the combination comprising: first means for sensing a furnace flame and second means for sensing the actuation voltage of said electrically operated fuel supplying means; solid state circuit means coupled to said first and second sensing means for detecting predetermined failure conditions and including a low frequency oscillator for providing a control signal representative thereof after a predetermined time delay; and lockout circuit means including controlled switch means coupled in series with the fuel supplying means, said lockout circuit means further including a control element coupled to said solid state circuit means and responsive to said control signal therefrom to actuate said controlled switch means for opening the operating current path of said fuel supplying means.
 6. The combination as defined in claim 5 wherein said controlled switch means comprise a relay with switch contacts held closed when current is applied to a relay coil associated with said relay during normal furnace operation, said switch contacts coupled in series with said fuel supplying means.
 7. The combination as defined in claim 6 wherein said control element comprises a transistor having base, collector and emitter terminals with said base terminal coupled to said solid state circuit means and said collector and emitter terminals coupled to opposite ends to said relay coil such that when said transistor is rendered conductive by said control signal applied to said base terminal, collector current flowing in said transistor shunts current from said relay coil opening said switch contacts in series with said fuel supplying means.
 8. For use in an oil furnace with a furnace burner, a burner motor and ignition means, a control circuit comprising: voltage detecting means coupled to said burner motor for detecting the existence of operating power applied thereto; flame detecting means associated with the furnace burner for providing a signal representing the existence or nonexistence of ignition; means for generating a heat command signal; a digital logic circuit coupled to said flame detecting means, to said voltage detecting means and to said generating means to provide a failure control signal after a predetermined delay period in the event a failure condition occurs; and lockout circuit means coupled to said burner motor and to said logic circuit for receiving signals from said logic circuit and responsive to failure control signals therefrom for interrupting the current path for said burner motor in the event a failure occurs.
 9. The control circuit as defined in claim 8 in combination with a thermostat switch for actuating a burner motor control circuit and wherein digital logic circuit includes: gate circuit means having a first input terminal coupled to said voltage detecting means, a second input terminal coupled to said flame detecting means, a third input terminal coupled to said burner motor control circuit and an output terminal for developing a pulse at said output terminal in the event a failure condition occurs; and pulse generating means including a control input terminal coupled to said output terminal of said gate circuit means for providing an output pulse for actuating said lockout circuit after a predetermined time delay upon receipt of a pulse from said gate circuit means.
 10. A protection circuit for preventing the operation of an ignition circuit associated with a furnace burner in the event a furnace control fails to supply the burner motor with operating power when heat is called for by the system''s thermostat, said circuit comprising: voltage sensing means coupled to said burner motor to sense the voltagE of the operating power supplied thereto; logic circuit means coupled to said voltage sensing means and to said furnace control for developing an output signal in the absence of power applied to said burner motor when said thermostat actuates to actuate said burner motor; and a lockout circuit coupled to said ignition circuit and to said logic circuit means and responsive to said output signal to deactuate said ignition circuit. 